This invention relates generally to a semiconductor wafer boat, and more particularly, to an improved semiconductor wafer boat for use in heat treatment of semiconductor wafers in a vertical furnace.
High temperature heat-treatment (annealing) of semiconductor wafers is commonly used to achieve certain desirable characteristics. For example, such a process may be used to create a defect free layer of silicon on the wafers. The high temperature annealing process of the type to which the present invention particularly relates is carried out in a vertical furnace which preferably subjects the wafers to temperatures above 1100 degrees C., and more preferably between about 1200 degrees C. and about 1300 degrees C.
During high temperature heat-treatment, at temperatures above 750 C and especially above 1100 C, the silicon wafers become more plastic. If the silicon wafers are not adequately supported during heat treatment, the wafers may undergo slip due to local gravitational and thermal stresses. As is well known in the art, slip may introduce contaminants into the device areas of the wafers. Moreover, excessive slip may lead to plastic deformation of the wafers, which in turn may lead to production problems, such as photolithography overlay failures causing yield losses in device manufacture.
Vertical wafer boats are used to adequately support semiconductor wafers, much like a rack, and, ideally, to minimize the local gravitational and thermal stresses on the wafers to avoid slip and plastic deformation while the wafers are being heat treated. A typical vertical wafer boat used in a vertical furnace comprises three or more vertical rails also referred to as rods. The rods typically have grooves or laterally extending fingers for supporting the wafers between the vertical rods within the boat. Each wafer may rest directly on fingers (or grooves) lying generally in a common horizontal plane. This configuration is common in the older art and is adequate when heat treating 200 mm and smaller diameter wafers. Alternatively, each wafer may rest on a wafer holder platform, e.g., a ring or solid plate, supported by the fingers (or in grooves), lying generally in a common horizontal plane. This configuration is common in the newer art and is usually necessary to adequately support 300 mm and larger diameter wafers. The 300 mm and larger diameter wafers are subjected to more local gravitational and thermal stresses than smaller diameter wafers, and the wafer holder platforms better support the 300 mm wafers by increasing the areas of the wafers that are supported.
Even with the use of wafer holder platforms, however, the 300 mm wafers may undergo slip and plastic deformation if the fingers and wafer holder platforms are not properly constructed. The fingers and the wafer holder platforms are subjected to local gravitational and thermal stresses in the furnace, just like the wafers. Unless the fingers and wafer holder platforms remain mechanically stable during heat treatment, they may undergo plastic deformation causing the wafers to undergo slip possibly leading to plastic deformation. The thickness of each finger and the thickness of each wafer holder platform correlate to the mechanical stability of the finger and wafer holder platform (i.e., the thicker the finger and the platform, the more mechanically stable they are). However, increasing the thickness of each finger and the thickness of each wafer holder platform decreases the number of wafers the boat can hold because of dimensional constraints imposed on the boat. For example, the wafer boat must have a height that fits within a vertical furnace and the wafers must be spaced apart in the boat a certain distance to allow a robotic arm to introduce and remove the wafers and wafer holder platforms. Increasing the thickness of each finger and the thickness of each wafer holder platform decreases the amount of vertical space available for the wafers. Additionally, thicker fingers and wafer holder platforms increase the thermal mass of the wafer boat which can also be detrimental to the slip performance of the boat. This is because of the possible increased stresses on the wafers resulting from higher thermal gradients that arise as a direct consequence of higher thermal mass. In general, the negative effects of increased thermal mass due to larger thicknesses of the fingers and wafer holder platforms can be overcome by reducing the temperature ramp rate during the annealing cycle.
Moreover, the wafer holder platforms must be properly supported by the fingers during heat treatment to substantially preclude plastic deformation in the platforms. In addition to the thickness of each finger, the positions of the fingers relative to the wafers correlate to the probability of plastic deformation of the wafer holder platforms. However, the positions of the fingers relative to the wafers are constrained by dimensional constraints of the boat. For example, the cross-section diameter of the boat must be such that the boat can fit within a furnace and at least two rods of the boat must be spaced apart a distance that allows a wafer, e.g., a 300 mm wafer, or wafer holder platform to be received between them.
With the thickness of each finger, the thickness of each wafer holder platform, and the positions of the fingers relative to the wafers constrained by the dimensions of the boat, the teachings of the prior art permit an adequately performing boat utilizing wafer holder platforms to hold up to about 90 wafers. Increasing the number of wafers that a wafer boat can hold and support without causing unsatisfactory slip and possibly plastic deformation in the wafers would increase the throughput of the furnace and reduce the cost of heat treatment per wafer.